Field of the Invention
This invention relates in general to the field of interferometry and, in particular, to a novel approach to providing a spectrally controlled source of interferometric light using time-averaged exposure.
Description of the Prior Art
Spectrally controlled interferometry (“SCI”) is a recently developed interferometric technique that allows implementation of white light interferometry (“WLI”) measurement schemes in common-path interferometers. See U.S. Pat. No. 8,422,026, U.S. Pat. No. 8,810,884 and U.S. Pat. No. 8,675,205, all hereby incorporated by reference. In contrast to conventional, high-coherence, laser interferometry, WLI is characterized by the absence of coherent noise because of the light's short coherence length, typically on the order of a few micrometers. Typically, coherent noise is one of the main sources of measurement errors in conventional interferometry.
Despite these difficulties, laser interferometry is extremely popular and useful because it allows the use of common-path interferometer designs—a particular class of devices in which most of the errors introduced by the optical system cancel out. This allows the manufacture of less expensive and more accurate instruments. High-coherence interferometry is also described as producing a non-localized interference pattern because the interference of beams occurs over a large volume of space, which is an advantage in setting up the measurement apparatus.
WLI is immune to the coherent-noise problems of laser interferometers but requires careful balancing of the optical path difference between the test and reference arm of the interferometer (OPD) so that interference can take place in the measurement space (i.e., within the coherence length of the light). Such arrangements can be complex and prevent the use of common-path interferometers, therefore forfeiting the above-described advantages. In the art, it is said that WLI produces localized interference because it is visible only in a limited volume around zero OPD.
SCI successfully combines both approaches and provides the advantages of both common-path interferometry and WLI. SCI produces localized interference in an unbalanced OPD interferometer and thus allows, for example, the use of a Fizeau interferometer in WLI mode, thus eliminating the problem of coherent noise. One of the major advantages of SCI is that existing instrumentation can be adapted to its modality of operation by replacing only the laser light source with one capable of proper spectral modulation. Therefore, such a light source and its proper modulation are of primary importance to the successful implementation of SCI.
To date, SCI has been implemented using broadband sources with filtering schemes (such as a Fabry-Perot etalon) to shape the source into a spectrally controlled source. These approaches suffer from loss of light and mechanical complexity, making them impractical. This disclosure is directed at a new concept for a source and a method of wide range spectral-modulation that is particularly suitable for SCI implementation because free of mechanical parts and, therefore, more robust and practical to implement.